Two of almost everything
Human body cells carry 46 chromosomes, but it is clearer to think of them as 23 pairs. The two members of a pair are called homologous chromosomes, or homologs. They are the same length, have their centromere in the same place, and carry the same genes in the same order along their length. You inherited one homolog of each pair from your mother and the other from your father.
Homologs carry the same genes, but not necessarily the same versions. At any given position — a locus — the two homologs may hold the same allele or different ones. That is the whole basis of inheritance: you have two copies of (almost) every gene, one per homolog, and which versions you carry shapes your traits.
Haploid, diploid, and why it matters
Ploidy is simply the number of complete chromosome sets in a cell. A cell with two matched sets — like your body cells, with their 23 pairs — is diploid, written 2n. A cell with a single set is haploid, written n. In humans, n = 23 and 2n = 46.
The one important exception is the gametes — egg and sperm. These are haploid, carrying just 23 chromosomes, one from each pair. The reason is arithmetic: when an egg and a sperm fuse, their two haploid sets combine to restore the diploid 46. If gametes were diploid, every generation would double the chromosome number. Halving first keeps the count stable across generations.
Counting through one human life cycle:
Body (somatic) cell ........ 2n = 46 (23 pairs, diploid)
|
| meiosis halves the set
v
Egg ........................ n = 23 (haploid)
Sperm ...................... n = 23 (haploid)
|
| fertilisation adds the two sets
v
Fertilised egg (zygote) .... 2n = 46 (diploid again)
n + n -> 2n
23 + 23 -> 46 ... the count is restored, not doubled.